Chapter 3: Stereoisomerism and Chirality – Study Notes

Chapter 3: Stereoisomerism and Chirality

Introduction to Stereochemistry

Stereochemistry is the study of the spatial arrangement of atoms in molecules and its influence on the physical and chemical properties of substances. This chapter focuses on the concepts of chirality, stereoisomerism, and their implications in organic chemistry.

Stereoisomerism

Types of Isomerism

• Structural (Constitutional) Isomers: Compounds with the same molecular formula but different connectivity of atoms.

• Stereoisomers: Compounds with the same connectivity but different spatial arrangement of atoms.

Type	Description
Constitutional Isomers	Same formula, different connectivity
Stereoisomers	Same connectivity, different spatial arrangement
Enantiomers	Nonsuperimposable mirror images
Diastereomers	Stereoisomers that are not mirror images

Chirality and Chiral Centers

Definition of Chirality

• Chiral: An object or molecule that is not superimposable on its mirror image.

• Achiral: An object or molecule that is superimposable on its mirror image.

The ultimate test of chirality is whether a molecule can be superimposed on its mirror image. If not, it is chiral.

Elements of Symmetry

• Plane of Symmetry: An imaginary plane dividing a molecule so that one half is the mirror image of the other.

• Center of Symmetry: A point in a molecule where identical components are located on opposite sides and equidistant from that point along any axis.

Chirality Center (Stereocenter)

• A chirality center (also called asymmetric center, stereogenic center, or stereocenter) is a tetrahedral atom, usually carbon, bonded to four different groups.

• All chiral centers are stereocenters, but not all stereocenters are chiral centers.

Example: A carbon atom with four different substituents (e.g., CHBrClF).

Enantiomers and Diastereomers

Enantiomers

• Non-superimposable mirror image isomers.

• Have identical physical and chemical properties in achiral environments, but differ in their interaction with other chiral substances and in the direction they rotate plane-polarized light.

Diastereomers

• Stereoisomers that are not mirror images of each other.

• Have different physical and chemical properties.

• Can be separated by standard chromatographic techniques.

Meso Compounds

• Molecules with chiral centers but also an internal plane of symmetry, making the overall molecule achiral.

• Example: Tartaric acid (meso form).

Number of Stereoisomers

• For a molecule with n chiral centers, the maximum number of stereoisomers is $2^n$.

Optical Activity

Definition

• A substance is optically active if it rotates the plane of polarized light.

• Chiral compounds are optically active if one enantiomer is present in excess.

Measuring Optical Activity: Polarimeter

• A polarimeter measures the rotation of plane-polarized light by a chiral compound.

Specific Rotation

• Defined as the observed rotation when a sample is placed in a tube 1.0 dm long at a concentration of 1 g/mL.

Formula:

$[\alpha]_D = \frac{\text{Observed rotation (degrees)}}{\text{Length (dm)} \times \text{Concentration (g/mL)}}$

• Dextrorotatory (d, +): Rotates light to the right.

• Levorotatory (l, -): Rotates light to the left.

Absolute Configuration: R/S System

Cahn-Ingold-Prelog Rules

• Assign priorities to substituents based on atomic number (higher atomic number = higher priority).

• For isotopes, use atomic mass to break ties.

• If still tied, move outward from the chiral center until a difference is found.

• Multiple bonds are treated as an equivalent number of single-bonded atoms.

Assign R (rectus, right) or S (sinister, left) configuration based on the order of substituents.

Chirality in Biological Systems

• Enzymes are chiral and interact differently with each enantiomer of a substrate.

• Enantiomers can have different physiological effects due to their interactions with other chiral molecules in living systems.

Example: One enantiomer of a molecule may have a citrus smell, while the other smells like pine.

Resolution of Enantiomers

• Racemic mixture: Contains equal amounts of both enantiomers; optically inactive.

• Resolution: The process of separating a racemic mixture into its individual enantiomers.

• Historically, Louis Pasteur first resolved enantiomers by separating asymmetric crystals of sodium ammonium tartrate.

• Modern methods include chiral chromatography.

Fischer Projections

• Used to represent three-dimensional molecules in two dimensions.

• Horizontal lines represent bonds projecting toward the viewer; vertical lines project away.

Stability of Isomers

• Stable at room temperature: Structural isomers, geometrical isomers, diastereomers, enantiomers.

• Unstable at room temperature (interconvert readily): Conformational isomers, inversion of amines.

Summary Table: Types of Isomerism

Type	Definition	Example
Structural Isomers	Same formula, different connectivity	Butane vs. isobutane
Stereoisomers	Same connectivity, different spatial arrangement	Cis-2-butene vs. trans-2-butene
Enantiomers	Non-superimposable mirror images	(R)- and (S)-lactic acid
Diastereomers	Not mirror images	Threose vs. erythrose
Meso Compounds	Chiral centers but overall achiral	Meso-tartaric acid

Additional info:

• For 1,2-disubstituted cyclohexanes, rapid ring interconversion allows them to be treated as planar for stereochemical analysis.

• Fischer projections are especially useful for carbohydrates and amino acids.